Counterbalances



April 1G, 1955 R. G. DE LA MATER 2,741,141

COUNTERBALANCES 5 Sheets-Sheet 1 Filed April 19, 1951 ATTORNEY April 10, 1956 R. G. DE LA MATER 2,741,141

COUNTERBALANCES l 3 Sheets-Sheet 2 Filed April 19 1951 INVENTOR April 10, 1956 R. G. DE A MATER 2,741,141

COUNTERBALANCES 3 Sheets-Sheet 3 Filed April 19, 1951 INVENTOR ATTORNEY CoUNrnRBALANCEs Robert Grilin De La Mater, Parkersburg, W. Va., assigner to The Parkersburg Rig & Reel Company, Parkersburg, W. Va., a corporation of West Virginia Application April 19, 1951, Serial No. 221,830

17 Claims. (Cl. M -591) This invention relates to counterbalances, and more particularly to a counterbalanced crank for well pumping rigs.

iIt is the common practice in the pumping of oil wells to support a walking beam intermediate its ends and to connect the pump rods to one end of the beam while the pitman of the pumping unit is connected to the other end of the beam to rock it on its axis. Moreover, it is the common practice with such a rig to transmit power to the pitman through a crank arm which is counterbalanced, usually by counterweights, for reasons which are so well known in the art as to require no discussion.

The counterweights commonly employed are so mounted on the crank arms as to provide for the adjustment of vthe weights toward and away from the axis of rotation to vary the effective counterbalancing action. To this end, the counterweights are commonly mounted on the cranks for sliding movement and the adjustment of the counterweights along the crank is a ditiicult and time-consuming operation because of the masses of the counterweights. In some installations and under some conditions, the means employed for clamping the counter- Yweights are loosened and the weights are struck `with sledge hammers to move them along the cranks, or pinch bars are used with substantial diliculty to accomplish the same result. The clamping means is then tightened to iix the position of the counterweights for the proper oper ation of the pumping unit.

An important object of the present invention is to pro vide a novel counterweight construction wherein the coun` terwieght is easily movable with minimum elort to adjusted positions along .the crank without the use of Sledge hammers, pinch bars or similar implements.

Afurther object is to provide a novel crank-counterweight combination having cooperating means providing for the relatively easy adjustment of the counterweight relative to the crank, and to provide means for xing the position ofthe counterweight quite eectively relative to the crank.

A further object is to provide a novel construction .of thecharacter referred to wherein the means `for adjusting the counterweight along the crank forms :part of a highly eiective means for vclamping the `counterweightY in ad- .justed position.

A further .object is to provide in such a construction a novel gear arrangement for effecting movement of the 1connterweight along the crank and to utilize the gear means for clamping the counterweight in :any adjusted position.

A further lobject .is to provide a novel crank-counter weight :combination wherein such elements are respectively lprovided with a rack and pinion, the latter of which is rotatable toletect :movement of the counter-weight, and `to provide novel means for utilizing the pinion for eiec tively ixing the counterweight in any adjusted position.

A further Objectis to provide suchen :apparatuswhereinthe pinion *bodily1 movable relative to the rack and 'has an operative 'position in which it is rotatable to ymove "ice the counterweight relative to the crank and has a second position in which the pinion is utilized for locking the counterweight against movement.

A further object is to provide such an apparatus having means associated therewith and cooperating with the pinion to move it to and hold it in its second position for clamping the counterweight in adjusted position and to utilize in such a construction a novel distribution of clamping forces for positively fixing the counterweight against movement.

Other objects and advantages of the invention will become apparent during the course of the following description.

In the drawings, I have shown one embodiment of the invention. In this showing,

Figure l is a side elevation of a pumping unit embodying the present invention,

Figure 2 is an end elevation of the crank showing the counterweights in position thereon, parts being broken away,

Figure 3 is an enlarged side elevation of the crank having the counterweights in position thereon, parts being broken away and parts being shown in section,

Figure 4 is a plan view of one of the counterweights,

Figure 5 is an enlarged fragmentary side elevation of a portion of a counterweight and a portion of the crank, parts being shown in section,

Figure 6 is a section taken on Vline 6-6 of Figure 3,

Figure 7 is a section taken on line 7-7 of Figure 5and Figure 8 is a fragmentary diagrammatic view showing the preferred relative arrangement of the locking pinions.

Referring to Figure l, a pumping unit is generally indicated as a whole and comprises a base or skid 10 on which is mounted a power source 11 connected vto a suitaole reduction unit 12 which may be of any desired form and drives a crank shaft 13 on which is mounted a novel type of crank indicated as a whole by the numeral 14. A wrist pin 16 is selectively connectible in a series of bearing .openings 17 in the crank and is connected to a pitman 18 actuating a Working beam 19 supported on a conventional Samson post 20. The other end of the walking beam is connected to the usual pump rod assembly 21.

As shown in Figure 3, the crank 14 tapers to ydecrease in width toward its free end, and its wider end is split as at .24 into the opening 25 in which the crank 13 is received. The split end of the crank is clamped by means of a bolt 26 to clamp the crank about the shaft 13, and the nuts 27 of the bolt 26 are arranged in recesses 23 extending through the crank. Each opening 28 is provided with inner and outer seats 29 and 30 respectively, eng'ageable by the nuts 27. These nuts engage the seats 29 toclarnp the crank about the shaft 13 and are movable outwardly against the seats to effect a positive spreading action of the split end of the crank to release it from 'the shaft 13. The crank, of course, is preferably keyed as at Y31 to the Ashaft 13 to form a positive driving Yconnection between these elements.

The upper and lower edges of the crank 14 are provided with racks 35. As shown in Figure 5, the teeth of the rack are true involute in form throughout the areas which .contact with the pinion teeth described below. The

' rack teeth deviate from conventional involue rack teeth 45. These counterweights are identical and only one need be described in detail. Referring to Figure 6, it will be noted that each counterweight in the edge thereof toward the crank is provided with an undercut channel 46 the top faces of whichrslope as at 47 to match the slope 39 of the adjacent crank dovetail 40. These sloping surfaces are adapted effectively to be clamped together in a manner to be described firmly to tix each counterweight in adjusted position. As shown in Figure 4, the opening in the top of each channel 46 is provided with sections of different widths. For example, spaced on opposite sides of the center of the length of each counterweight, such opening is slightly wider as at 50 while the remaining portions are slightly narrower as at 51. Such portions l of the channel have their lower sloping faces slightly higher than the faces 47 as at 52 (Figure 6). The purpose of this arrangement of parts is to provide clearance for substantial distances along the adjacent portions of the crank and counterweight to facilitatermovement of the counterweights by the means described below. The channel 46 isr widened at its ends as at 53 and such widened portion at the radially outer end of the counterweight terminates in an inner shoulder 54for a purpose to be described. l

Each counterweight is provided in the edge thereof corresponding with the channel 46 with a recess Y55 opening into the associated channel 46 (Figures 3 and 6) and a pinion 56 is arranged in each such recessrto mesh with the associated rack 35. The teeth of the-pinions are preferably true involutes throughout their areas which come into normal rolling contact with the rack teeth.v However, as shown, the bottom fillets are muchV Ylarger than standard and the tips are rounded. Each pinion is mounted on a shaft 57, -one end of which is polygonal as at 58 for engagement with a wrench. v The faces of the counterweights adjacent such polygonal end are Vrecessed as at 59 for reception of the socket of the wrench. Particular attention is invited to the fact that the shafts of the pinions operate in slots 61, wider than the shafts, and inclined as shown in Figures 3 and 5 with respect to the adjacent edges of the crank. These openings slope away from the rack 35 toward the adjacent edges of the counterweights for a reason which will become apparent.

In each recess 55 is arranged a shoe 65 pivoted to the associated counterweight as at 66 for movement into and out of engagement with the adjacent pinion. The inner surface of each shoe 65 is curved to correspond to the curvature of the associated pinion so that when the shoe is moved toward the adjacent pinion, it will engage several teeth thereof.

Each shoe 65 is adapted to be actuated by a bolt 67 threaded in a nut 68 mounted in a recess 69. The turning of the head 70 of each bolt moves the associated shoe 65 into engagement with one of the pinions or releases it for movement out of engagement with such pinion. It'will become apparent that when each pinion shaft is in the end of its slot 61 farthest from the associated rack 35, the'pinion is in proper meshing relation with the rack so that turning movement of a pinion will' eect movement of the associated counterweight along the crank. A cotter pin on each bolt 67, indicated by the numeral 72, prevents the loss of these bolts after they have been installed. To facilitate access to the head 70 of each bolt 67, the adjacent edge of the associated counterweight is recessed as at 73 (Figure 7).

As further referred to below, the tightening of each bolt 67 moves the associated pinion 56 to move its shaft 57 along the slot 6l relatively toward the crank. This action develops certain hnes of force three of which are shown in Figure 6 and indicated by the numerals 75, 76 ,and77. Other lines of force are developed, for example with respect to the lines 78 and .79 in Figure 5.

The counterweights are particularly adapted to have d auxiliary counterweights connected thereto when an additional counterweighting action is desired. To this end,

each counterweight 45 is adapted to have connected thereto a counterweight indicated asia whole by the numeral (Figures l, 2 and 3). It will be noted that the edges of the counterweights 45 remote from the shaft 13 are arcuate and approximately concentric with the axis of the shaft 13. The counterweights 85 are provided with edges 86 forming continuations of such arcuate edges. Each auxiliary counterweight 85 is provided with a pair of bolt openings 88 inclined toward each other in the direction of the crank 14 as shown in Figure 3. Each such bolt opening receives a bolt 89 the nut 90 of which is arranged in a recess 91 to be protected and yet accessible to a wrench.

The adjacent edges of each counterweight and its associated auxiliary counterweight are provided respectively with a recess 94 and a` pad 95. The bolt opening 88 extends through the pad 95 and the latter seats in the associated recess 94, and it will become apparent that the bolts` 89 are thus relieved of all shearing stresses. Opposite edges of each counterweight are provided with a narrow slot 97, of a width to receive the shankof the bolt 89. The upper end of the slot-97 is widened as at 98 similarly to receive the head 99 of the bolt, and the inner extremities of the slots 97 and 98 are provided with a recess 100 into which the head 990i the bolt is adapted to drop as in Figure 3. Each auxiliary counterweight 85 is provided with an opening 102 therethrough for the reception of a lifting bar.

Edge portions of the crankl 14 are recessed on site sides as at 104 to receive stop plates 105 bolted V:in position as at 106. Two sets of the recesses 104 are shown in Figure 3, and eitherset may be usedpas will become apparent. The plates 105 are engageablc against'the shoulders 54 (Figure 4) as explainedV below.

Operation The crank 14 is easily placed in position by backing ott' the nuts 27 into engagement with the pads or seats 30 to expand the split 24. Thus the crank may be placed in position, whereupon the nuts 27 are tightened against the seats 29 to clamp the split end of the crank about the shaft 13. The wrist pin 16 is conventionally connected in either of the openings 17, depending upon the particular operating conditions.

The counterweights 45 are assembledy on the crank prior to the installation of the stop plates 105 and bolts 106, the grooves or channels 38 thus opening freely through the free end of the crank. The shoes 65 are released from the pinions 56 to permit the latter freely to turn. With the parts in the positions shown in Figure 3, the lower counterweight is moved into positionV fromthe end of the crank. Upon engagement of the pinions 56 of such counterweight with the lower rack 3:5,` the pinions will start toturn, and thepinions, of course, will be free to rotate. As soon as there is meshing engagement between the pinion teeth and the rack teeth,YV the pinions may lbe rotated to move the counterweight along the crank. To accomplish' this, the left hand shaft 57 of the counterweight as viewed in Figure 3 will be rotated'by asuitable wrench, the pinion being turned in a clockwise direction. By rotating such pinion to move the counterweight toward the left in Figure 3, the reaction forces on the pinion will maintain the pinion shaft in the end of the slot 61 remote from the rack 35. The wrench should extend to the right in Figure .3 to be moved downwardly to accomplish this resuit. Thus there'will be no tendency for the pinion to move from a position in whichv there is operative meshing of the pinion and rack.

When the proper adjusted position of the thus applied counterweight is reached, the locking means will be brought, into operation. VThe crank .is then rotated and the other counterweight is then applied in a oppo- Y similar manner to the then lower edge YofV the'crank. It may be pointed out that the crank may be calibrated and a pointer provided on cach counterweight to facilitate moving each counterweight to adjusted position. The recesses 104, of which there may be any desired number, represent a plurality of adjusted positions of the cranks. With the cranks in the positions shown in Figure 3, the stop plates 105 will engage the shoulders 54 of the cranks. if greater counterbalancing action is desired, the 'stop plates 1175 will be arranged in the radially outer recesses 104. In their positions shown in Figure 3, the counter-weights cannot be extendedbeyond the outermost radius of swing of the crank. However, the counterweights may be extended beyond the crank to provide more effective counterbalancing for installations where the unit is mounted high enough to permit this additional radius of swing'.

When a counterweight is to be locked in position, a wrench is applied to the head 70 of each bolt 67. This operation and its eects will be considered in coniunction with Figures and 6. As the bolt 67 is tightened, the associated pinion shaft 57 will be moved upwardly to the left in the slot 61 in Figure 5. This action causes the pinion teeth to become crowded against or wedged relative to the teeth of the rack 35. If one pinion tooth is directly above center of its shaft 57, as viewed in Figure 5, such pinion tooth will become wedged in the associated bottom iillet of the rack bar 35. This eiects an upward reaction, against the rack bar and a downward reaction on the counterweight, thus wedging the opposite shoulders 39 and 47 (Figure 6) tightly against each other.

The force thus exerted by the pinion on the crank When the counterweights are to be moved, the crank is moved to Yhorizontal position as in Figure 5, whereupon the bolt 67 ofthe lower counterweight (Figure 5) is backed off to release'the shoe 65 from the pinion. The pinion then may be turned to effect positive movement of such counterweight along the crank in the manner described in connection with the applicationV of each counterweight to the crank and its movement to adjusted position. If the lower counterweight is' to be moved to the right as viewed in Figures 3 and 5, the right hand pinion will be used for the reason described above in connection with the left hand pinions. The right hand pinion of the lower counterweight will be turned counterclockwise. After such counterweight has been adjusted, the crank will be rotated and the other counterweight similarly adjusted. This, of course, is merely suggestive as to the easiest manner for eifecting rotational movement of the pinions.

The slotted arrangement for the auxiliary counterweight attaching bolts is provided for the convenient attachment of additional counterweighting in the iield, if

desired.

As to the means for locking the counterweights, it will be noted that since there is not proper operative meshing of the pinion teeth and rack in the clamped position of the parts, tooth interference will prevent rotation of the is across the width of the pinion, and such vertical force is indicated in Figure 6 by the line 77. This line bisects the lines of force, indicated by the lines 75 and 76, om the sloping bearing surfaces 39 and 47. This results in a Wedging action and the distribution of forces is such as to cause the weight to align itself properly in the vertical plane of the crank.

Referring to Figure 5, it will be noted that inward movement of the bolt 67 clamps the shoe 65 against several teeth of the pinion 56, thus distributing the forces around several pinion teeth instead of `concentrating the force on one tooth. This pressure between the shoe 65 and the pinion not only moves the pinion upwardly to exert the forces previously referred to, but in addition, the shoe 65 acts as a brake tending eiectively to prevent rotation. of the pinion, thus further adding to the effectiveness of the locking action.

When the screw 67 is initially tightened, the shaft 57 slides upwardly along the bottom of the slot 61 until the end of the slot is reached, whereupon the shaft engages the end portion of the slot along a line which varies Somewhat according to tolerances between the shaft and slots. GenerallyV speaking, this line will be in the plane of the line 79 (Figure 5). Assuming that one of the pinion teeth is at top center, such tooth will contact with the root of the rack substantially coincident with the line Y78 which will be approximately vertical. Continued tightening of the bolt 67 applies a force in the line of the axis of the bolt, and this Aforce will be divided :into two components generally represented by the lines 7S and 79. The exact locations of the lines 7S and 79 will depend upon the intel-engagement of the pinion and rack teeth and the line of engagement of the shaft 57 with the end portion of the slot 6i, but as indicated above, the lines 78 and 7.9, `for practical purposes, may be considered to be vertical and horizontal respectively. The component of force indicated by the line 78 has its reaction from the rack, and the force indicated by the line 79 has its reaction vfrom the vpoint of bearing between the shaft and slot, these two .reactions balancing the force applied by the screw.

pinion. ln the second place, the pressure exerted by the pinion tooth at the root of the rack tooth draws the tapered dovetail fit into tight weoging engagement. As a third function, the friction resulting from the clamping of the shoe 65 against the teeth or the pinion prevents the rotation of the latter, and as a fourth function, friction resulting from pressure between the pinion shaft and its elongated opening 61 tends to hold the vparte in position. It fulther will be noted that any tendency for a counterweight to move will tend to move one pinion with it,thus drawing the shaft of such pinion further up in the tapered slot and more tightly wedging it. Thus there are numerous functional advantages in the structure shown. Accordingly, the present construction provides not only means for positively moving the counterweights without Sledge hammers, pinch bars, etc., but also provides means for effectively holding the counterv/eights in adjusted positions.

The foregoing description covers the function of the various parts without regard to the relative arrangement of the two pinions carried by each counterweight. For maximum eiciency, the pinions of each weight are preferably arranged to have a denite relationship to each other. lt will be apparent that if the two pinions of each weight are arranged in the same relative positions, then it will be possible for both pinions to stop in positions corresponding to the position of the left hand pinion in Figure 8 with one tooth of each pinion crowded against a bottom llet of the rack. This would involve a single line or contact between one tooth of each pinion and the rack and would not afford as highly eilcienta locking action as would be true if both pinions stopped in positions corresponding to the position of the right hand pinion in Figure 8 with a definite wedging action occurring between one tooth of the rack and two-teeth of each pinto rhe ideal conditions of operation would involve Varranging the two pinions 'of each counterweight in corresponding positions and stopping both pinions of each weight in positions corresponding to theright hand pinion in-Figure 8. As a practical matter, it Awould be impossible lto accomplish this. In practice, therefore, .the spacing of the axes 'of the pinions Vis preferably some integral multiple of the tooth pitch plus one-half pitch. This-'wouldntrange the pinions relative to each other as shown in Figure 8 in which case :if a connterweight is located on -the crank so that one pinion is in one-tooth engagement with the brake, the other ,pinion will bein the more eifective two-tooth engagement. Under such conditions, there will be no pinion positionsrin which theminimum locking action would occur. This arrangement guarantees the greatest possible locking effect regardless of the relative positions of the crank and either counterweight.

l I claim: Y l

l. A counterbalanced structure comprising an arm supported for turning movement on an ya'xislperpendicular to the length thereof, a counter-weight carried oy said arm, means for effecting movement of said counter-weight toward and away from said axis to vary the counterbalancing action, and means for effecting movement of said firstnamed Vmeans into clamping engagement with said arm to tix said counterweight against said movement, said second-named means being bodily movable with said rstnamed means and engageable therewith in any position of said counterw'eight along said arm.

2. A Vcounterbalanced structure comprising an arm supported for turning movement on an 'colis perpendicular to the length thereof, a counterweight carried by said arm, means for effecting movement of said counterweight toward and away from said axis to vary the counterbalancing action, said means being supported by said counterweig'ht into clamping engagement therewith to x said counterweight against said movement relative to said arm.

3. A counterbalanced structure comprising an arm mounted for' turning movement on an'axis perpendicular to theV length thereof, a counterweight connected to said arm for sliding movement longitudindlyV thereof Ytoward and awayfrom said axis, said arm being provided with a rack, Va pinion carried by said counterweight and engaging said rack whereby rotation of said pinion will move saidrcounterweight longitudinally of said arm, said pinion being mounted for movement toward and away from said rack, and means for moving said pinion toward said rack to eect clampingrengagement of said pinion with said rack to fix said counterweight against movement along said arm.

4. A counterbalanced structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight connected to said arm for sliding movement longitudinally thereof toward and away from said axis, said arm being provided with a rack, a pinion carried by said counterweight and engaging said rack whereby rotation of said pinion will move said ccunterweight longitudinally of said arm, said pinion being mounted for movement toward and away from said rack, Va shoe movable into engagement with said pinion generally in the direction of movement of said pinion toward said rack whereby, when said shoe is moved in such direction, it brakes said pinion and moves the latter into clamping engagement with said rack to iix said counterweight against movement along said arm.

5.u A counter-balanced structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight connected to said arm for sliding movement longitudinally thereof toward Yand away from said axis, said arm being provided with a rack, a pinion anda shaft on which it is mounted, said counterweight Vhaving slots receiving the ends of said shaft and Vsaid slots being inclined relative to said rack whereby said pinion is relatively movable toward and away from said rack, said pinion infone position relative to said racl;

Vhaving normal meshing engagement therewith, and means for moving said pinion toward said rack into clamping engagement therewith to prevent m vement of said counterweight along said arm.

6. Apparatus constructed in accordance with claim wherein the means for moving said pinion toward said rack comprisesra member carried by said counterweight and movable into engagement with said pinion, and means for moving said member generally longitudinally of said slots into engagement with said pinion to eiect movement thereof into clamping engagement with said rack.

7. Apparatus constructed in accordance with claim 5 wherein themeans for moving said pinion toward said rack comprises a shoe pivotally connected to said counterweight Vand havingan arcuate pinion-engaging surface corresponding in curvature to the teeth of said pinion so that when it isr swung toward said pinion, it will engage several teeth thereof, and screw means for moving said shoe into engagement with said pinion to move the latter into clamping engagement with said rack.

8. A counterweight structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight arranged at one edge of said arm, said arm and said counterweight having inter-engaging surfaces spaced from said edge for slidably connecting said counterweight to said arm for movement longitudinally of said arm, said interengaging surfaces diverging transversely of said arm and in the direction of said counterweight, means for eecting movement of said counterweight longitudinally of said arm, and means carried by said` counterweight for effecting a force againsttsaid edge of said arm to create reaction forces to wedge said interengaging surfaces against each other to prevent movement of said Vcounterweight along said arm.

9. A counterweight structure comprising an arm mounted for turning movement onganV axis perpendicular to the length thereoha counterweight arranged at one edge of said arm, said arm and said counterweight having interengaging surfaces spaced from said edge for slidably connecting `said counterweight tor said arm for movement t longitudinally of said arm, said interengaging surfaces divergingV transversely of said arm and in the direction of said counterweight, said arm being provided with a rack along said edge thereof, a pinion carried by said counterweight outwardly of said edge of said arm and engaging said rack whereby, upon rotation of said pinion, said counterweight will be moved along said arm, and means carried by said counterweight for transmitting a forceragainst said edge of said arm to generate an opposite reaction forcewedging said interengaging surfaces against each other to prevent movement of said counterweight along said arm.

10. A counterweight structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight arranged at one edge of said arm, said arm and said counter-weight having interengaging surfaces for slidably connecting said counterweight to said arm for movement longitudinally of said arm, said interengaging surfaces diverging transversely of said arm and in the direction of said counterweight, said arm being provided with a rack along said edge thereof, a pinion carried by said counterweight outwardly of said edge of said arm and engaging said edge of said arm comprises a brake shoe carried by said counterweight and engageable with the teeth of said pinion, and means for moving said brake shoe into braking engagement with the teeth of said pinion and to move said pinion into clamping engagement with said rack.

12. Apparatus constructed in accordance with claim 10 wherein the means for moving said pinion toward said rack comprises a brake shoe pivotally supported at one end by said counterweight and having an arcuate surface engageable simultaneously with several teeth of said pinion, and screw means threaded in said counterweight and engageable with said brake shoe to move it into braking engagement with said pinion and to move the latter into clamping engagement with said rack.

13. A counterweight structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight arranged at one edge of said arm, said arm and said counterweight having interengaging surfaces for slidably connecting said counterweight to said arm for movement longitudinally of said arm, said interengaging surfaces diverging transversely of said arm and in the direction of said counterweight, said counterweight having slots inclined relative to said edge of said arm, a pinion, a shaft carrying Vsaid pinion and projecting from opposite sides thereof and slidable in said slots whereby said pinion is movable toward and away from said rack, said pinion, when in a predetermined relation to said rack, being rotatable to effect movement of said counterweight along said arm, and means for transmitting a force to said pinion to move it toward said edge of said arm into clamping engagement with said rack and to create an opposite reaction force to wedge said interengaging faces against each other to x said counterweight against movement along said arm.

14. Apparatus constructed in accordance with claim 13 wherein the means for forcing said pinion into clamping engagement with said rack comprises a brake shoe pivoted to said counterweight for swinging movement into engagement with teeth of said pinion, and screw means threaded in said counterweight and engageable with said screw to move it into engagement with said pinion.

15. A counterweight structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight slidably connected to said arm for movement toward and away from said axis, a rack carried by one edge of said arm, said counterweight being provided at each of two spaced points longitudinally of said rack with a pair of parallel slots, said pairs of slots being inclined toward each other in the direction of said edge of said arm, a pair of pinions each mounted on a shaft projecting from opposite sides'thereof and each shaft being slidable in one pair of said slots, each pinion being movable toward and away from said rack upon sliding movement of its shaft in the associated slots, and each pinion, when in a predetermined position relative to said rack, being rotatable in meshing engagement with said rack to move said counterweight longitu- 10 dinally of said arm, and means for forcing each pinion into clamping engagement with said rack to iiX said counterweight against movement longitudinally of said arm.

16. Apparatus constructed in accordance with claim l5 wherein said arm and said counterweight are provided with interengaging faces connecting said counterweight to said arm for sliding movement therealong, said interengaging faces diverging toward said edge of said arm whereby, when either or both of said pinions are forced toward said edge of said arm to etect clamping engagement between said pinions and said rack, an opposite reaction force will wedge said interengaging faces against each other and fix said counterweight against movement longitudinally of said arm.

17. A counterbalanced structure comprising an arm mounted for turning movement on an axis perpendicular to the length thereof, a counterweight connected to said arm for sliding movement longitudinally thereof toward and away from said axis, said arm being provided with a rack, a pair of pinions carried by said counterweight and engaging said rack whereby rotation of either pinion will move said counterweight longitudinally of said arm, each pinion being mounted for movement toward and away from said rack, and means for moving each pinion toward said rack to effect clamping engagement of each pinion with said rack to fix said counterweight against movement along said arm, said rack extending continuously between and beyond said pinions and the axes of said pinions, when said pinions are clamped against said rack, being spaced apart a distance approximately equal to an integral multiple of the pitch of said rack teeth plus one-half pitch.

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